any animal having a backbone or spinal column. Verbrates can be traced back to the Silurian period. In the adults of nearly all forms the backbone consists of a series of vertebrae. All vertebrates belong to the subphylum Vertebrata of the phylum Chordata. There are five classes of vertebrates: fish, amphibians, reptiles, birds, and mammals. General characteristics of vertebrate animals include their comparatively large size, the high degree of specialization of parts they exhibit, their bilaterally symmetrical structure, and their wide distribution over the earth. In addition to an internal skeleton of bone and cartilage or of cartilage alone, vertebrates have a spinal cord, a brain enclosed in a cranium, a closed circulatory system, and a heart divided into two, three, or four chambers. Most have two pairs of appendages that are variously modified as fins, limbs, or wings in the different classes. All animals without backbones are called invertebrates; these do not form a homogeneous group as do vertebrates.

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